1. Field of the Invention
The present invention relates to a cylindrical storage battery provided with a spirally coiled electrode plate assembly. More particular, it relates to an alkaline storage battery which is suitable for, for example, the driving power source of vehicles. The battery is improved in the shape of the current collectors welded to the upper and lower parts of the electrode plate assembly to secure the welding of the spiral coiling start part of the electrode plate assembly with the current collector and thereby to permit high-rate charge and discharge through the current collectors.
2. Related Art
Various types of cylindrical alkaline storage batteries are known, representatives of which include nickel-cadmium storage batteries and nickel-metal hydride storage batteries. These batteries are in wide use for various equipment, such as portable telephones and notebook-sized personal computers, because of their high reliability and easy maintenance. In recent years, the development of cylindrical storage batteries suited to high-rate discharge are eagerly awaited as the power source for motor power assist bicycles, lawn mowers and further electric automobiles and the like.
Such a cylindrical storage battery for high-rate discharge has an electrode plate assembly comprising one strip-shaped positive electrode plate and one strip-shaped negative electrode plate which are wound as spiral coils with a separator interposed therebetween, and the assembly is housed in a battery case made of metal. As to the structure for collecting the output and input currents from or to the electrode plates suited to a large electric current, a type in which respectively one rectangular or nearly circular plate-formed current collector is welded at a plurality of spots to the terminal edge of respective one-side electrode plate projecting outward respectively from the upper and lower end planes of the electrode plate assembly (this type being hereinafter referred to as tabless type) is disclosed, for example, in U.S. Pat. Nos. 3,732,124 and 5,238,757, and is in general use at present.
Various types have been proposed for such current collectors welded to the terminal part of the electrode plate assembly, a representative one of which is the current collector 14 shown in FIG. 7.
This current collector is provided with a rectangular flat plate part 14a, rib-formed projections 14b formed by bending vertically downward the two side edges stretching along the length direction of the plate part 14a, and two U-shaped cutout parts 14c or punched holes. The welding of the current collector 14 to the electrode plate assembly is effected through electric resistance welding by making the rib-formed projections 14b cross the terminal edge of one side electrode projecting upward from the electrode plate assembly, pressing a pair of bar-formed welding electrodes-against the upper side of the flat plate parts 14d of the current collector 14 which hold the cutout part 14c therebetween or against the upper side of the projection 14b, and passing electricity between the electrodes under application of pressure.
In the above-mentioned current collector 14, however, the rib-formed projection 14b is formed in the direction parallel to both side edges which extend along the length direction of the plate part 14a. Accordingly, even when welding is conducted by pressing a pair of welding electrodes against the plate part 14a located at the positions which hold the cutout part 14c therebetween and passing electricity under applied pressure, the electric current which flows on the plate part 14a between the pair of welding electrodes (namely, loss current in welding) is large owing to adverse conditions of the distance of electricity passage and the electric resistance on the contrary, the electric current which flows through the crossing part of the pair of projections 14b and the electrode terminal edge is small, so that it is difficult to melt the crossing part of the projection 14b and the electrode terminal edge sufficiently to secure good welding.
Resultantly, the weld part, namely the integrated part formed by the rib-formed projection 14b and the electrode terminal edge, has a high contact resistance and, when the battery is discharged with a large electric current, for example, 3c (electric current of three times the battery capacity), the IR (voltage) drop at the weld part is too large (resulting in loss of electricity) to give a satisfactory battery performance.